The invention pertains to rolling element bearings, and, in particular, to such bearings which are used at high speeds and heavy loads.
That rolling element bearings are effective in reducing the friction between a rotating shaft and a fixed journal, or between a rotating wheel and a fixed axle, is amply demonstrated by the widespread usage of such bearings in an enormous variety of applications. Such bearings are sometimes identified as antifriction bearings because there is generally far less friction associated with the rolling action between the rolling elements and the inner and outer races than with the sliding action between a shaft and journal in a journal bearing.
Rolling element bearings vary in size, rotational speed, operating load and temperature, and type of rolling element. The most widely used types of rolling elements in such bearings are balls, cylindrical rollers and frustoconical tapered rollers.
Many factors combine to determine the limiting speeds of bearings. To provide a check on bearing speed limits, size and rotational speed are conveniently combined in a quantity called "DN", which is the product of inside (or bore) diameter of the inner race (in millimeters) times the rotational speed (in revolutions per minute). It has been observed that bearings having a DN value above about 1,500,000 seem vulnerable to problems such as sliding or skidding of rolling elements, overheating, rolling contact fatigue, and race fracture due to high hoop stresses. Thus, a DN value over about 1,500,000 arbitrarily, but effectively, defines a high speed bearing.
Numerous means to reduce wear in rolling element bearings have been devised. For example, rolling elements and both races are frequently made of hardened steel. The steels normally used contain high levels of carbon, generally above about 0.5 percent, or the surfaces of the components of such bearings are carburized to similar carbon levels. Takei et al. (Japanese Unexamined Patent No. JP 58-174718A) have disclosed the application of hard coatings to rolling elements and both races as a means for reducing wear. However, it has been discovered that hard coatings on the rolling elements or the races may be detrimental to the operation of heavily loaded high speed bearings. Under such conditions, the hard coatings may crack or spall off, producing loose particles of the hard material. Those loose particles act as contaminant particles, as discussed below.
Rolling element bearings typically include a spacer means, usually called a cage, separator or retainer, which serves to space the rolling elements from each other uniformly around the periphery of the races. For some bearings designed to operate at high values of DN, the cage is guided by having it operate in close proximity to land or shoulder regions on either the inner race or the outer race. There is a close radial clearance, on the order of 0.010 inch, between the cage and the land regions of the guiding race. This design keeps the cage more nearly coaxial with the bearing, which improves the uniformity of spacing of the rolling elements, and also reduces vibration or wobbling of the cage.
However, a disadvantage of this design is that hard contaminant particles may become trapped in the clearance region between the cage and land region of the guiding race; if so, the contaminant particles may cause added wear of the bearing components. The problem is particularly severe when the cage is made of a relatively soft material such as bronze or when it is coated by a soft anti-seize coating such as silver. The contaminant particles may become embedded in this soft material. Such embedded contaminant particles act to cut into the land region of the guiding race. Further, debris particles produced in the wear process behave to contribute to the wear process. In spite of the considerable care normally exercised to prevent contamination of a bearing, either during manufacture, assembly or operation, such contamination remains a problem to the designer and user of rolling element bearings.
Bearings having races made of steel having very fine carbide particles in its microstructure, such as bearing races made by powder metallurgy techniques, are especially vulnerable to the problem of accelerated wear from contamination and debris because the individual carbide particles are too small to prevent contaminant and debris particles from lifting the carbide particles and supporting matrix from the surface of the metal.
Rotating compressor and turbine components of aircraft gas turbine engines are typically supported in rolling element bearings. Such bearings are routinely subjected to high speeds, high temperatures and high operating loads, yet they must be kept as light as possible to minimize overall engine weight. DN values of about 2,000,000 are typical. Engine operating temperatures dictate the use of tool steels such as M50 in place of widely used bearing steels such as AISI 52100. The bearings in an aircraft gas turbine engine must carry the entire thrust generated by the engine, plus loads resulting from the weight of the rotating components plus loads generated by in-flight maneuvers, air turbulence and landings. Such high speed, highly loaded bearings are critical to operation of the engine. Thus, such bearings must be designed and manufactured to minimize the likelihood of bearing failure.